Methods of and apparatus for reproducing images by transferring electrostatic charges

ABSTRACT

Documents are reproduced on a continuous strip of paper by transferring from a photoconductive belt to the paper an array of electrostatic charges, configured to conform to images on the documents. Both the paper and the photoconductive belt are trained around metallic drums which have identical peripheral speeds. The drums are juxtaposed so that they essentially bring the paper and photoconductive belt into contact along a line lying in a plane whiich is tangent to both drums. When the electrostatic charges on the belt arrive at this line, they are transferred to the paper because there is a potential difference existing between the drums due to corona charging of the drum carrying the photoconductive belt and grounding of the drum carrying the paper. The charges, of course, retain the line configuration of the image as they are transferred from the belt to the paper. The capacitance existing between the drums with the photoconductive belt and paper varies due to environmental conditions, such as temperature and humidity, and varies due to lack of compatibility between the photoconductive layer and the paper. This variation in capacitance causes imperfections in images transferred to the paper. In order to compensate for variations caused by this capacitance, the two drums are connected through a circuit which contains an auxiliary capacitor that has a capacitance at least 100 times greater than the capacitance existing between the drums. Since the capacitance of this auxiliary capacitor is so high, it renders the capacitance existing between the two drums insignificant in relation to the total capacitance thereby, in essence, rendering the capacitance independent of both environmental conditions and incompatibility between the paper and photoconductive belt.

United States Patent 1 Quang METHODS OF AND APPARATUS FOR REPRODUCING IMAGES BY TRANSFERRING ELECTROSTATIC CHARGES [75] Inventor: Pham Kim Quang, Dieppe, France [73] Assignee: La Cellophane, Paris, France [22] Filed: Dec. 3, 1973 [21] Appl. No.: 421,041

[30] Foreign Application Priority Data Primary Examiner-Mary F. Kelley Assistant Examiner-Judson R. Hightower Attorney, Agent, or Firm--Sherman & Shalloway ABSTRACT Documents are reproduced on a continuous strip of paper by transferring from a photoconductive belt to the paper an array of electrostatic charges, configured 51 Dec. 23, 1975 to conform to images on the documents. Both the paper and the photoconductive belt are trained around metallic drums which have identical peripheral speeds. The drums are juxtaposed so that they essentially bring the paper and photoconductive belt into contact along a line lying in a plane whiich is tangent to both drums. When the electrostatic charges on the belt arrive at this line, they are transferred to the paper because there is a potential difference existing between the drums due to corona charging of the drum carrying the photoconductive belt and grounding of the drum carrying the paper. The charges, of course, retain the line configuration of the image as they are transferred from the belt to the paper. The capacitance existing between the drums with the photoconductive belt and paper varies due to environmental conditions, such as temperature and humidity, and varies due to lack of compatibility between the photoconductive layer and the paper. This variation in capacitance causes imperfections in images transferred to the paper. In order to compensate for variations caused by this capacitance, the two drums are connected through a circuit which contains an auxiliary capacitor that has a capacitance at least 100 times greater than the capacitance existing between the drums. Since the capacitance of this auxiliary capacitor is so high, it renders the capacitance existing between the two drums insignificant in relation to the total capacitance thereby, in essence, rendering the capacitance independent of both environmental conditions and incompatibility between the paper and photoconductive belt.

3 Claims, 1 Drawing Figure U.S. Patent Dec. 23, 1975 METHODS OF AND APPARATUS FOR REPRODUCING IMAGES BY TRANSFERRING ELECTROSTATIC CHARGES BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to methods of and apparatus for transferring and permanently recording images and, more particularly, this invention relates to methods of and apparatus for reproducing documents by electrostatically transferring images of those documents to a dielectric surface, such as a sheet of paper.

2. Prior Art and Technical Considerations It is well-known how to reptoduce documents by transferring to a strip of paper a configuration of electrostatic charges which are arrayed to conform to images on the documents'This is accomplished by forming the image of electrostatic charges on a photoconductive belt and then bringing the belt into line contact with the paper. Since the paper is a dielectric, the charges leave the photoconductive belt and adhere to the paper along the line of contact, thereby forming charge images on the paper. The paper now carrying the image of electrostatic charges is advanced over a developer which applies a toner to the paper. The toner is distributed over the paper in accordance with the density of the charges, thereby creating a visible image. This visible image is permanently fixed on the paper by applying heat thereto.

Generally, the afore-described process is most conveniently accomplished by training the paper and the photoconductive belt about peripherally juxtaposed drums made of conductive material. The drums are generally of the same diameter and are rotated at the same peripheral or tangential speed. In order to cause electrostatic charges to transfer from the belt and adhere to the paper, a potential difference is created between the drums by charging one drum and grounding the other so that as the photoconductive belt and paper come into line contact, the ionization threshold between the two is exceeded, causing transfer of the electrostatic charges.

While a photoconductive belt may be used in some instances, it is also a general practice to simply make the surface of the drum not carrying the paper photoconductive so that a photoconductive belt need not be used.

It has been found that the general afore-described process is deficient in that the reproductions achieved thereby vary in quality. This is due to both environmental conditions, such as fluctuations in temperature and humidity, and to lack of compatibility and homogeneity between the photoconductive and dielectric surfaces.

One of the reasons why quality of reproduced images is affected by these conditions is that a variable capacitance is created between the drums. In effect, there is a capacitive circuit created between the two drums due to the layered effect of the photoconductive belt and dielectric paper being sandwiched therebetween along the line of contact. The circuit formed thereby is equivalent to one which would be formed by a capacitance having a high value leakage resistance connected in parallel between the drums.

The drum that carries the photoconductive surface or has a photoconductive layer is charged by a coronacharging device to a value Q C V where C is the capacitance existing between the drums and V is the potential difference created between the drums. The charge Q is limited by leakage resistance, but the value of V is variable because of variations in C, due to the aforementioned environmental variations and to imperfections in the compatibility between the photoconductive and dielectric surfaces. These variations manifest themselves by undesirable discharges which create distortion in line contact as the electrostatic charges are transferred from the photoconductive surface to the paper.

SUMMARY OF THE INVENTION In view of the afore-mentioned considerations, it is an object of this invention to provide new and improved methods of and apparatus for reproducing images.

Another object of this invention is to provide new and improved methods of and apparatus for reproducing images, wherein the reproduction thereof is unaffected by environmental conditions, such as temperature and humidity.

Still another object of the instant invention is to provide new and improved methods of and apparatus for reproducing images, wherein the reproduction thereof is independent of the homogeneity or compatibility between a photoconductive surface carrying an array of charges conforming to the image and a dielectric surface receiving an array of charges.

It is still another object of the instant invention to transfer an image of electrostatic charges from a photoconductive surface to a dielectric surface by charging an electrode supporting the photoconductive surface with a corona-charging device while grounding an electrode carrying the dielectric surface.

It is a further object of this invention to provide methods of and apparatus for transferring an image of electrostatic charges from a photoconductive surface to a dielectric surface by creating a potential difference between electrodes carrying these surfaces while connecting the electrodes through an auxiliary capacitor which overrides the effects of capacitance naturally existing between the electrodes.

It is an additional object of this invention to provide new and improved methods of and apparatus for transferring an image of electronic charges from a photoconductive surface to a dielectric surface by carrying the surfaces on electrodes, one of which is charged by a corona-charging device and the other of which is grounded, wherein the electrodes are connected by a circuit having an auxiliary capacitor, the capacitance of which is greater than the capacitance existing between the two electrodes.

In accordance with these and other objects, a method Q? practicing the instant invention includes forming images of documents or the like on a photoconductive surface while advancing the photoconductive surface over a first electrode. The image is then transferred to an uncharged dielectric surface while the uncharged dielectric surface is in contact with a second electrode, juxtaposed with the first electrode, in order to bring the photoconductive surface and the uncharged surface into proximity with one another, to thereby transfer the image to the uncharged surface. This transfer is effected by creating a potential difference between the electrodes by charging the first electrode with a corona-charging device while grounding the second electrode. In order to eliminate environmental or atmospheric interference with transfer of the image, a cir- 3 cuit including an auxiliary capacitor is connected between the two electrodes to form a circuit parallel with the two electrodes. This auxiliary capacitor has the capacitance which is substantially higher than the capacitance existing between the electrodes.

An apparatus for practicing the instant invention includes first and second electrode means for advancing a photoconductive surface and a dielectric surface respectively, wherein an image of electrostatic charges is formed on the photoconductive surface for transfer to the dielectric surface. A corona-charging device is utilized to bring the first electrode to saturation potential and thereby create a potential difference between the first and second electrodes. Since a capacitance exists between the electrodes, a parallel capacitive circuit is connected between them. This capacitive circuit includes an auxiliary capacitor having a capacitance which is substantially higher than the capacitance existing between the electrodes.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic illustration of an apparatus embodying the principles of the invention which may be used to practice the method of the invention.

DETAILED DESCRIPTION Referring now specifically to the drawing, there is shown a document 11 to be reproduced which, in the illustrated embodiment, is advanced from a supply reel 12 to a take-up reel 13. If the document happens to be a negative, light may be passed therethrough, as shown by the arrows l4-l4. If the document is a print, then light will be reflected therefrom in a conventional manner. In any case, light either passed through or reflected from the document is focused by lens 16 to produce an image of the document on a photoconductive surface 17. In the illustrated embodiment, the photoconductive surface is a belt trained around a pair of drums l8 and 19.

The photoconductive surface may utilize any one of a number of photoconductive materials. For example, the belt may be in the strip of either paper or aluminum covered with a standard photoconductive layer having a base of zinc oxide, polyvinyl carbazole, cadium sulfide, or any other known photoconductive substance. While the illustrated embodiment uses a belt, the drum [8 may provide a photoconductive surface in lieu of the belt by coating the drum with one of the afore-mentioned photoconductive substances or another known photoconductive substance. The photoconductive belt 17 is initially charged upstream of the area of image impingement by a corona-charging device 2] which distributes an array of electrostatic charges 22-22 on the belt. As the charges 22-22 are advanced downstream through the image impinged on the belt 17, they are rearranged according to the density of light patterns on the belt. Since the light patterns are arranged according to the focused image, the charges 22-22 will arrange themselves according to the focused image. In other words, if the document being reproduced is a negative, then more of the particles will congregate in the relatively light areas or relatively illuminated areas than in the less illuminated areas. Consequently, when the image is finally fixed, the light areas will appear dark and the dark areas will appear light.

After the charges 22-22 are arranged on photoconductive belts 17 in accordance with the image appearing on document 11, the charge is transferred to a strip of paper 26 which is being advanced from a supply reel 27 to a take-up reel 28 around a metallic drum 29. The paper 26 is preferably dielectric paper which is, initially at least, uncharged. In the illustrated embodiment. the drum 29 has the same diameter as the drum l8 and is rotated in the opposite direction at the same peripheral or tangential speed as the drum 18. This counter-rotation is accomplished in some conventional manner by using, for example, a conventional gear train. As seen in the drawing, the drums are juxtaposed in close proximity to one another with a gap 31 therebetween, which is just wide enough to accommodate both the photoconductive belt 17 and the strip of paper 26. As the photoconductive belt 17 and the paper 26 traverse the narrowest portion of the gap 31, they come into virtual contact with one another, allowing the charges 22-22 to transfer from the photoconductive belt to the paper, as will be explained hereinafter.

The drum 18 is charged by a corona-charging device 32 which, in the illustrated embodiment, is a pointed metal probe which is spaced a distance of a few millimeters to two centimeters from the drum 18. The corona-charging device 32 may be powered by a voltage supply 33 which energizes both the corona-charging device 32 and the corona-charging device 21 through a conventional controller 34. In practice, the coronacharging device 32, by being brought to a potential of 5,000 to 30,000 volts, can induce a potential of between 200 to 3,000 volts in the drum l8. It should be kept in mind that, instead of using a pointed probe such as the device 32, a wire placed parallel to the drum 18 can also be used.

In order to create a potential difference between the drum l8 and the drum 29, the drum 29 is grounded by a lead 37. The potential difference thus created between the drums l8, and 29 causes the electrostatic charges 22-22 to transfer from the photoconductive belt 17 to the paper 26 as the paper and photoconductive belt come into virtual contact in the gap 31. This contact is generally line contact so that the charges 22-22 will register in successive lines on the paper 26. The successive lines ultitpately create an image of electrostatic charges on the paper 26.

As or after the image of electrostatic charges is transferred to the paper 26, the paper passes over a developer 38 which, in the illustrated embodiment, includes a roller 39 that engages the paper and distributes a toner 41 onto the paper. The toner 41, of course, distributes itself across the paper in accordance with the density of the electrostatic charges 2222. In order to fix the image on the paper 26, a heater 32 dries the toner and adheres the toner to the paper before the paper is stored on take-up reel 28.

As previously discussed in the BACKGROUND OF THE" INVENTION, there is between the metallic drums l8 and 29, which are separated by photoconductive belt [7 and the dielectric paper 26, a capacitance. This capacitance is varied by atmospheric conditions, such as temperature and humidity, and by any lack of compatibility or homogeneity between the paper and photoconductive belt. As this capacitance varies, the quality of the image of electrostatic charges transferred from the photoconductive belt to the paper is varied, causing distorted or unclear final images on the paper. In order to alleviate the effects of this varying capacitance, a circuit, illustrated generally by the line 45, is connected between the drums l8 and 29 in parallel relationship to the capacitance occurring at gap 31.

This circuit 45 includes a large capacitance illustrated by the capacitor 46.

ln practice, the capacitor 46 will have a capacitance which is at least 100 times that of the capacitance occurring at gap 31. Generally, the capacitance at gap 31 will be in the order of I pico farads, while the value of the capacitor 46 will generally be between 10,000 and l00,000 pico farads or, rather, from to 100 nano farads. Consequently, the variations in the capacitor 31 due to atmospheric conditions and lack of homogeneity between the paper 26 and the photoconductive belt 17 will be inconsequential in relation to the total capacitance existing between the drums l8 and 29. This is because the total capacitance is orders of magnitude greater than any variation in capacitance caused by these conditions.

Since compatibility and homegeneity between the paper 26 and the photoconductive belt 17 is rendered relatively inconsequential, the paper 26 can be of ordinary quality. Although it is perhaps desirable to make the paper of dielectric material, even paper which has conductive properties may be used. The paper may be coated with a layer of dielectric resin of any nature. For example, it may have a base of vinyl polymers, silicones, polystyrenes or styrene-butadiene copolymers. In addition, pigments may be used to improve the appearance of the paper, such pigments being zinc sulfide, silica, titanium oxide, zinc oxide or other known materials.

I claim:

I. A method of reproducing an image on an uncharged surface, comprising the steps of:

forming the image of electrostatic charges on an intermediate photoconductive surface;

advancing the photoconductive surface while in contact with a first electrode; advancing the uncharged surface while in contact with a second electrode juxtaposed with the first electrode to bring the uncharged surface and photoconductive surface into proximity with one another so that there is virtual contact therebetween;

creating a potential difference between the electrodes by charging the first electrode with a coron0-Charging device while grounding the second electrode, to thereby transfer the image of electrostatic charges to the uncharged surface, said potential difference clue to the capacitance determined by the interaction of the electrodes, photoconductive surface and uncharged surface; and

connecting the electrodes with a circuit which includes an auxiliary capacitor having a capacitance substantially higher than the capacitance existing between the electrodes.

2. The method of claim 1, wherein the first and second capacitors are cyclindrical drums of conductive material and wherein the drums are rotated at the same tangential speed and are separated by a gap which is just wide enough to accommodate the photoconductive surface and uncharged surface in virtual contact with one another in order to transfer the image of electrostatic charges.

3. The method of claim I, wherein the capacitance of the auxiliary capacitor is at least one-hundred times higher than the capacitance existing between the first and second electrodes in order to render the capacitance between the first and second electrodes insignificant relative to total capacitance.

i IF i i 

1. A METHOD OF REPRODUCING AN IMAGE ON AN UNCHARGED SURFACE, COMPRISING THE STEPS OF: FORMING THE IMAGE OF ELECTROSTATIC CHARGES ON AN INTERMEDIATE PHOTOCONDUCTIVE SURFACE;
 2. The method of claim 1, wherein the first and second capacitors are cyclindrical drums of conductive material and wherein the drums are rotated at the same tangential speed and are separated by a gap which is just wide enough to accommodate the photoconductive surface and uncharged surface in virtual contact with one another in order to transfer the image of electrostatic charges.
 3. The method of claim 1, wherein the capacitance of the auxiliary capacitor is at least one-hundred times higher than the capacitance existing between the first and second electrodes in order to render the capacitance between the first and second electrodes insignificant relative to total capacitance. 